Influence of air pollutants on allergic sensitization: the paradox of increased allergies and decreased resistance to infection.

Air pollution has long been associated with health risks such as increased susceptibility to respiratory infections and potentiation of asthmatic-type responses. Experimental evidence in rodents indicates that air pollutants including diesel exhaust particles (DEPs), gases, and metals cause lung injury, inflammation, reduce aspects of host defense, and may potentiate allergic airway responses. Here we present evidence that diesel exhaust particles delivered by inhalation or aspiration can exacerbate allergic lung disease depending on the material's chemical properties. Genomic analysis of mouse lungs following instillation or inhalation of DEPs shows an alteration spectrum of pathways associated with immune signaling, cell metabolism, and oxidative stress. Diesel exposure also may worsen respiratory infections through depression of protective immune responses. Here we show that mice exposed to diesel and co-infected with influenza had increased influenza virus titers as well as higher levels of lung injury and inflammation in association with increased Th2 cytokines, and a concomitant decrease in Th1 polarization. A simplified model explains how the potentiation of the Th2 arm of immunity by diesel exhaust results in increased allergic sensitization, whereas cell-mediated (protective) immunity against viral infections is simultaneously reduced.